Note: Descriptions are shown in the official language in which they were submitted.
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AIRFOIL MASKING TOOL AND METHOD OF POLISHING AN AIRFOIL
Field of the Invention
The invention relates to an airfoil masking tool and use of the masking tool
in a method
of polishing the airfoil.
Background of the Invention
lo The demand for ever greater efficiency gains in gas turbine engines has
lead to the
demand for ultra-fine (low surface roughness) airfoils that have a surface
roughness Ra
in the region of 1 to 5 micro-inches. NASA has demonstrated that an industry-
standard
surface finished compressor rotor blade ultrapolished, also known by super
finishing,
super polishing, ultra finishing and high precision surface finishing, to a 5
micro-inch
finish can produce an increase in engine efficiency of approximately 0.5%,
William B.
Roberts et al, The Effect of Ultrapolish on a Transonic Axial Rotor, ASME
Turbo Expo
2005 International Gas Turbine and Aeroengine Congress Reno Nevada, June 6 to
9,
2005.
It is widely known that media finishing processes, such as those recipes that
are
commonly provided with media finishing equipment sold by the Rosier, Sweco,
Giant,
Royson, etc., are able to polish most metal surfaces to achieve surface
roughness Ra
measurements in the region of 7 to 25 micro-inches. The media finishing
process
typically comprises a tub style, batch bowl, or a continuous flow-through
vibratory
finisher filled with hard ceramic media stones of various shapes, abrasive
content and
sizes, that is vibrated with an electric motor that spins an eccentric weight.
Hard
ceramic media is loaded into the bowl and the act of vibrating the bowl causes
that
media to flow in a directional manner and circulate around the bowl. Water and
burnishing compounds are typically added to the bowl to assist in the
polishing, and
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sometimes a paste or powder may also be added to accelerate the process. The
articles that are to be polished are added to the bowl so that they flow
around with the
media. The parts can also be fixed in a stationary position in the bowl, but
this is not
typical. An example of a suitable polishing machine is shown in U.S. Patent
No.
6,261,154.
High energy finishing processes such as high energy tumbling or centrifugal
finishing
and drag-finishing are able to achieve lower surface finish conditions.
However, the high
energy nature of these processes can result in the loss of material at sharp
edges which
may harm the dimensions of the part.
When it comes to polishing close-toleranced parts such as gas turbine engine
airfoils,
the polishing process can be very aggressive on sharp radius edges and corners
such
as the leading and trailing edges of the airfoils and blade tip corners.
Changes in the
dimensions of the leading and trailing edges and blade tip corners can have a
profoundly detrimental effect on the mechanical properties and aerodynamic
efficiency
of the airfoils. Thus, a process for super-polishing close-toleranced airfoils
must be able
to preserve the dimensions of these areas and possibly others.
Summary of the Invention
An objective of the invention is to provide a super-polishing media process
that will
avoid altering close-toleranced dimensions of parts such as turbine blades.
Another objective is to provide an airfoil masking tool constructed to hold
and protect
parts of the airfoil during the polishing process.
The objectives can be obtained by a method of polishing an airfoil comprising:
mounting an airfoil in a masking tool, the masking tool covering at least
one of a leading edge, trailing edge or tip of the airfoil, to provide a
mounted
airfoil;
placing the mounted airfoil in a polishing machine;
polishing the mounted airfoil by contacting an exposed surface of the
airfoil with a polishing medium at a flow angle that provides a surface
roughness
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Ra of less than 5 micro-inches, to form a polished airfoil having a surface
roughness Ra of less than 5 micro-inches; and
removing the polished airfoil from the masking tool.
The objectives can also be obtained by using an airfoil masking tool
constructed to hold
an airfoil during polishing comprising:
a body constructed and arranged to hold an airfoil in place during polishing
and
to cover at least one of a leading edge, trailing edge or tip of the airfoil,
or any edges or
surfaces of the airfoil that are to be protected from abrasion.
Brief Description of the Drawings
Fig. 1 illustrates a masking tool.
Fig. 2 illustrates the airfoil masking tools ganged together in a row.
Fig. 3 illustrates a base plate.
Fig. 4 illustrates a bladed disc or rotor.
Figs. 5 and 6 illustrate a bladed disc or rotor with masking tooling.
Fig. 7 illustrates a graph of the results of an erosion test.
Fig. 8 illustrates an erosion test procedure.
Fig. 9 illustrates a polishing machine.
Fig. 10 illustrates a vane sector with masking tooling
Detailed Description of the Invention
The invention will now be explained with reference to the attached non-
limiting Figs.
Fig. 1 illustrates a masking tool 7 designed to hold parts, in this case
airfoils 1, during
polishing. The tool 7 comprises a body 5 constructed to cover at least the
leading edge
2 or trailing edge 3 or blade tip 4 of the airfoil 1, so that the polishing
media contacts the
exposed surface of the airfoil and cannot directly contact the leading edge 2
or trailing
edge 3 or blade tip 4. The airfoil 1 can be secured in the body 5 by means of
an end
cap 6 to hold the airfoil root and also prevent polishing in this area. The
body 5 and end
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cap 6 can be made of metal such as but not limited to steel, titanium,
aluminum or
nickel alloys or non-metallic materials such as, but not limited to rubber of
varying
hardness or plastic such as ABS, Nylon, reinforced Nylon, polycarbonate,
polypropylene, DeIran or a combination of the above. The masking tools 7 can
be
designed so that there is minimal wear and material loss on the masking tools
7 so that
they can be used multiple times. The airfoil 1 can have a coating present on
the
exposed surface. This coating can be applied by physical vapor deposition
methods.
As shown in Fig. 2, a plurality of airfoil masking tools 8 may be ganged
together in a row
9 and then placed over a plurality of airfoils 10 to be polished. The
plurality of airfoils 10
can be located in one of the slots of a rail 11. The masking tool assembly
fits so that
each airfoil leading edge, trailing edge and tip can be automatically aligned
with the
associated masking tool 8. Preferably, the exposed surface of the airfoils to
be polished
should be aligned so that the flow of polishing medium contacts the surfaces
at the
same angle between the medium flow direction and the orientation of the
leading
edge/trailing edge chord axis of the airfoils; termed the flow angle.
The rail of blades 12 or individual masking tools 7 can then be fitted onto a
base plate
13 as shown in Fig. 3. Multiple rails of blades 12 that can be of the same
size and
shape, but may also be of different part designs that can be loaded into
adjacent slots
on a base plate 13. The fully loaded base plate 13 can then be secured in the
polishing
machine, for example a tumbling machine. Equally, the blades may be organized
in
other patterns, such as a curve or staggered arrangement, as an alternative to
the linear
arrangement shown in Fig. 3. Additionally, the masking tooling may be secured
on a
ferromagnetic base plate by means of a magnetic component to hold the tooling
and
blade in the correct position without the need for a rail. Additionally,
blades may be
mounted into other tooling structures more suited to the type of polishing
machine to be
used as exemplified but not limited to tumbling or drag finishing machines.
Bladed discs or rotors 14, as shown in Fig. 4, are well known engine
components that
comprises of airfoils 15 that are integral to the rotor hub. The leading 16
and trailing
edges 17 and blade tips 18 can be protected using masking tooling 19, 20, as
shown in
Fig. 5 and as a complete assembly 21 in Fig. 6 in order to prevent excessive
material
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removal during the tumbling process. The masking tooling can be made from
metal
such as but not limited to steel, titanium, aluminum or nickel alloys or non-
metallic
materials, such as, but not limited to rubber of varying hardness' or plastic
such as ABS,
Nylon, reinforced Nylon, polycarbonate, polypropylene, DeIran or a combination
of the
above.
Vane sectors 27, as shown in Fig 10, are well known engine components that
comprise
airfoils 22, an outer shroud 23, and an inner shroud 24 into which the
airfoils are
attached. The leading 25 and trailing 26 edges of the airfoils can be
protected by using
a masking tool 30 comprising of two parts; and upper 29 and a lower 28 part;
and as a
complete assembly 31 in Fig. 10 in order to prevent excessive material removal
during
the polishing process. The masking tooling can be made from metal such as but
not
limited to steel, titanium, aluminum or nickel alloys or non-metallic
materials, such as,
but not limited to rubber of varying hardness' or plastic such as ABS, Nylon,
reinforced
Nylon, polycarbonate, polypropylene, DeIran or a combination of the above.
The present invention can utilize any suitable polishing machine for mass
finishing the
surface of workpieces, in particular the airfoil masking tool holding the
airfoil. Fig. 9
illustrates an exemplary embodiment of a suitable polishing machine. The
polishing
machine comprises a container or tub 100 which Fig. 9 illustrates as being
circular or
toroidal in its shape, and which¨in this and related shapes¨is referred to as
a "bowl."
zo In its dictionary definition, the term "toroid" refers to "a surface
generated by a plane
closed curved rotated about a line that lies in the same plane as the curve
but does not
intersect it" (Merriam-Webster's Collegiate Dictionary, 10th Edition, 1993).
The shape is
more colloquially referred to as resembling a doughnut. It will be understood
that
although a toroid is the best method of describing the shape of this
embodiment of the
bowl 100, that the invention is not limited to this particular shape nor
should the term
"toroid" as used herein, be limited to structures that meet the rigorous
mathematical
definition. Those familiar with solid geometry and the like will of course
recognize that
the functional equivalent of a toroid could be made using slightly different
shapes, but
that these would fall within the claims of the invention. Other container
shapes that can
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be used with the present invention include, but are not limited to, troughs,
ovals, and
racetrack shapes.
The tub 100 holds a finishing media which is generally designated by the
dotted
portions 112. The finishing media is a collection of small objects, usually
selected to be
uniform in shape, size, and composition, which strike a workpiece to be
finished and
carry out a polishing or abrading action upon it. The nature and type of
finishing media
selected for use with the invention is not critical to the invention, but
exemplary media
include natural stone, sand, porcelain, ceramic particles of various shapes
and sizes,
metal balls, certain natural organic media (e.g. walnut shells), or polymer-
based
materials or hybrid multi-component media (e.g. plastic or porcelain with
embedded
abrasive particles such as diamond). The individual pieces of the media are
also
referred to as "working bodies" to differentiate them from the workpieces
being finished.
In Fig. 9, the workpiece 113 to be polished is illustrated as the open wheel
113. It will be
understood that although a simple open wheel is illustrated, the invention
offers
significant advantages for workpieces of much more complex shape, as shown by
the
airfoil in the attached Figs., and that the simple illustration of Fig. 9 is
included for
schematic and illustrative purposes rather than as any limitation of the
claimed
invention.
The invention further comprises means for moving the media 112 in the tub 100
in a
generally revolving motion that is indicated by the arrow 114 in Fig. 9. The
control of the
media 112 in the tub 100 is generally well understood in this art and will not
be
discussed in detail herein. Exemplary discussions of the manner in which the
motion of
the tub 100 can be used to move the media 112 are set forth, for example, in
U.S. Pat.
No. 3,464,674 at Column 3, line 26 though Column 4, line 38, and U.S. Pat. No.
4,428,161.
For example, a motor can be flexibly mounted to the tub and an eccentrically-
mounted
weight on a motor shaft can be used for vibrating the motor and the tub when
vibrations
are desired.
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One embodiment of the invention is shown in Fig. 9, which utilizes a
positioning and
rotating device, examples shown as the rotating shaft or spindle 121, for
positioning and
rotating the workpiece 113 that is to be polished in the media 112. The shaft
121
may rotate or hold the workpiece 113 stationary about an axis 124 that is
oblique to the
axis 122 about which the media revolves, and does so without moving the
position of
the workpiece 113 with respect to the tub 100 as the workpiece 113 is held or
rotated.
The workpiece 113 can be made to hold stationary or rotate the workpiece at
any angle
to the axis 124 to produce the best desired orientation for polishing the
workpiece 113.
Instead of using the positioning and rotating device, the workpiece can be
mounted in a
fixed position inside the tub 100.
In addition to the two non-limiting examples of polishing machines disclosed
herein,
other polishing machines can be used. The invention is applicable to any
polishing
machine capable of adjusting the angle of the flow of the polishing media in
relation to
the workpiece being polished. By specifically aligning the airfoils and
protecting the
leading edge, trailing edge and tip, the exposed surfaces of the airfoils can
be polished
to higher degree. Preferred polishing machines are a tumbling machine, a high
energy
centrifugal barrel finishing machine or a drag finishing machine. A preferred
medium is
ceramic. The polishing machine should be constructed to flow the medium with
or
without an abrasive paste at desired flow angles against the exposed surfaces
of the
airfoils. Preferably, the flow angle is selected to provide a surface
roughness Ra of less
than 5 micro-inches. Examples of suitable flow angles are 50 to 0 degrees,
more
preferably 40 to 10 degrees, and most preferably 20 to 10 degrees, to the
orientation of
the leading edge/trailing edge chord axis of the airfoils.
In tumbling machines having two side vibration motors, one can be set at 0 to
50
degrees, and more preferably +10 to 40 degrees, and more preferably +10 to 20
degrees and the other side motor at 0 to -50 degrees, and more preferably -10
to -40
degrees, and more preferably -10 to -20 degrees. However the motor orientation
can
be altered to change the flow angle of media as necessary such that the flow
angle is
within 50 to 0 degrees and more preferably 40 to 10 degrees, more preferably
30 to 10
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and most preferably 20 to 10 degrees at the desired angle to the orientation
of the
leading edge/trailing edge chord axis of the airfoils.
Bladed discs or rotors 14, as shown in Fig. 4, are well known engine
components that
comprises of airfoils 15 that are integral to the rotor hub. The leading 16
and trailing
edges 17 and blade tips 18 can be protected using masking tooling 19, 20, as
shown in
Fig. 5 and as a complete assembly 21 in Fig. 6 in order to prevent excessive
material
removal during the tumbling process. The masking tooling can be made from
metal
such as but not limited to steel, titanium, aluminum or nickel alloys or non-
metallic
materials, such as, but not limited to rubber of varying hardness' or plastic
such as ABS,
Nylon, reinforced Nylon, polycarbonate, polypropylene, DeIran or a combination
of the
above.
A preferred medium for polishing metallic airfoils comprises ceramic media,
such as the
RCP porcelain non-abrasive polishing stones that can be acquired from ROsler
along
with a Rosier RPP6279 abrasive paste. However, these media are usually not
suitable
for polishing airfoils that are coated with an erosion resistant coating such
as
BalckGolde. Surprisingly, a method that was found to produce a surface finish
to levels
below 4 pin was a medium comprising diamond paste. The paste used to polish
the
BlackGolde coating was comprised of a one-micron diamond powder with a gum
that
serves to keep the diamond powder on the surface of the ceramic media and a
water
soluble oil, commonly used in metallographic polishing, that assists in the
acceleration
of the polishing process.
Preferably the polishing paste comprises a polishing media and a carrier. The
polishing
media can be any media suitable for polishing an airfoil. Examples of suitable
media
include, but are not limited to, ceramic and diamond. Any suitable carrier for
the media
can be used. Preferred carriers comprise gum, water and oil.
A preferred polishing paste comprises the following components:
at least one gum in the range of 4 to 24 mL, preferably 8 to 16 mL, more
preferably 10 to 13 mL;
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at least one water soluble oil in the range of 26 to 104 mL, preferably 26 to
78
mL, and more preferably 45 to 65 mL;
water in the amount of 1 to 3 L; preferably 1 to 2 L and more preferably 1 to
1.6
L;
at least one ceramic media, with the amounts being per 100kg of ceramic media.
The amounts of the components can be adjusted up and down within these ranges
for
any desired amount of ceramic media. When polishing a coated airfoil, the
polishing
paste preferably further comprises at least one diamond powder in the range of
26 to
156 grams, preferably 52 to 104 grams, and more preferably 65 to 78 grams.
Examples of suitable polishing paste compositions comprise:
Diamond powder in the range of 100 to 600 grams, preferably 200 to 400 grams
and more preferably 250 to 300 grams;
Gum in the range of 15 to 90 mL, preferably 30 to 60 mL and more preferably 40
to 50 mL;
Water soluble oil in the range of 100 to 400 mL, preferably 100 to 300 mL, and
more preferably 150 to 200 mL;
Water in the range of 3 to 10 L, preferably 4 to 7 Land more preferably 4 to 5
L;
and
ROsler RCP media in the range of 200 to 600 kg, preferably 300 to 500 kg and
more preferably 360 to 410 kg.
The invention is also suitable for fine adjustments to a structure of the
airfoil or other
desired workpiece. For example, the polishing can be conducted to remove a
desired
portion of the airfoil to change or alter a dimension or shape of the airfoil.
For example,
the airfoil can be machined or cast into a desired shape and then fine
adjustments to
the shape can be performed at the same time as polishing, by controlling the
flow of
media over the surface of the part such that the action of the media is more
heavily
concentrated in the area where a dimensional adjustment is required. The
surface of
any desired portion of the airfoil can be removed at the same time as
polishing. This
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method is suitable for controlled removal of material ranging from 1 micron up
to one
millimeter in thickness of material from the airfoil.
The polishing method will be further described with reference to the following
non-
limiting examples.
Examples
The process for the super-finishing of parts such as turbine blades comprises
of the
following components:
Example 1
1. Tumbling machine
The example of the tumbling machine used in this embodiment of the process
was a Walter Trowal MV-25
2. Ceramic media
The ceramic media used in this process can be almost any media that is
suitable
for contacting all areas of the part to be polished. One embodiment of this
process used Rosier RCP porcelain non-abrasive polishing stones to process the
parts.
3. An abrasive paste
The abrasive used in this process comprises:
2.5Kg Rosier paste (RPP6279), or Rosier RPP579, or Walther Trowel SDB
Trowapast PKP
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5L water
And was a suitable quantity to use with 800-900 lbs Rosier RCP media.
4. Stationary fixed parts
Airfoils protected with masking tooling similar to that described here were
mounted on a base plate and loaded into the tumbling machine and were held
stationary on a plate in the tumbler as shown in Fig. 3.
The Walter Trowal MV-25 tumbling machine is equipped with three vibrator
motors; two
on the side and one on the base. The two side motors can be oriented
individually
about 360 degrees. In the present example, the two side motors were set to 10
degrees from the horizontal; one at +10 degrees and the other at -10 degrees.
During operation the three motors were set to 100% power. The media flows in
one
direction, for example generally from the leading edge to trailing edge of the
airfoils, and
every 14 minutes the medium flow was reversed automatically by the machine so
that
the medium flow direction was generally from trailing edge to leading edge and
then
from leading edge to trailing edge. This cycle was repeated for 5 to 51/2
hours. Longer
or shorter time periods can be used as required to achieve the required
surface finish.
Once the polishing run was completed the media parts were rinsed with water
and a 2-
5% by volume of a burnishing compound (brand name Rosier FC120) for 45 minutes
to
an hour. At this point the process was complete and the polished parts were
removed
from the media. The surface roughness Ra was less than 5 micro-inches.
Example 2
The same process as Example 1 was used to super polish airfoils that were
first coated
with an erosion resistant coating, MDS Coating Technologies' BlackGold
coating. The
erosion resistant coating was applied to the airfoils and once polished
according to the
present invention to a surface finish (Ra) of less than 4 pin. The surface
finish retention
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of the coated and polished surface was compared to an uncoated surface having
a
surface finish (Ra) of less than 4 pin by subjecting the polished coated and
uncoated
surfaces to erosion using Arizona road dust as the abrasive media. Fig. 7
illustrates the
results of the erosion test. The results shown in Fig. 7 demonstrate that the
polished
coating prolonged and maintained the surface finish in erosive conditions to
an Ra of
less than 10 pin. In contrast, the uncoated polished surface at the same
conditions
resulted in a surface finish Ra of 34 pin. The erosion test procedure is shown
schematically in Fig 8.
The abrasive paste for polishing coated gas turbine blades (Example 1, Item 3)
is:
275g of 1 micron diamond powder
45mL xanthan gum
200mL water soluble oil ¨ Anamet Rust Inhibitor
4 - 5 L water
And was a suitable quantity to use with 360 ¨410 kg Rosier RCP media.
While the claimed invention has been described in detail and with reference to
specific
embodiments thereof, it will be apparent to one of ordinary skill in the art
that various
changes and modifications can be made to the claimed invention without
departing from
the spirit and scope thereof.
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